Representation of amphibolites in compositional space

F)MASH

Total 15.233 Total 15.233

5.3 Representation of amphibolites in compositional space

foliation

di¡ection

in

some samples whereas later, cross-cutting or

infilling

chlorite is

associated

with

altered or fractured hornblende (e.g. Fig. 5.12), biotite or garnet or in cross- cutting quartz

veins.

Fine grained white mica (either muscovite, or more often ^a paragonite- dominated Solid solution, Table 5.2) defines the

foliation

direction

in

a number of samples.

It

also occurs ^as acicular inclusions in plagioclase which may exhibil a very contorted geometry (Fig.

5.9).

Coarser grained white mica may be parallel to the

foliation

or, more commonly, cross-cuts the.foliation.

Cummingtonite,generally forms ^f,rne to rnedium epitaxial grains ^cjn hornblende @ig.'

S.

i¡).

^Epidote (PsS-¡o) occurs as inclusions in most phases and forms part of most matrix asSociations as relatively coarse, euhedral grains or very fine anhedral grains in large aggegates.

It

is generaily parallel to the

foliation

and shows zonal birefringence due to compositional zoning and slight to

sÍong

pleochroism. Coarse epidote grains ^are

discontinuously zoned, from pale green, high birefringence epidote

in

the core to colourless, lower birefringence ^(less Fe.rich) epidote in the

rim

(Table

5.2).

Chemical va¡iation in epidote due to zonation (from clinozoisite-rich cores to epidote-rich rims) may be as inuch as 20 moia¡

Vo. Ilmeníte often dehnes inclusion trails

in

garnet and hornblende, however the dominant oxide in.the

m?Fx

is

rutile.

Euhedrat magnetite is very.iarely present in ^the matrix and as

inclusions.

Ankerito or, ralely, calcite occurs in ^the matrix as relatively coarse subhedral to anhedral grains.

It

is generally the most Mg-riCh phase present with

Xp",4¡¡

in the range

0.lg-0.27

^(Table

5.2).

The order of Fe-enrichment amongst the phases in these assemblages is consistent anrl increases

in

the order ankerite (Xpe,,qnt 0.19'-0.21) < chlorite (Xps,ç¡1 0.24- 0.46)

< biotite

(Xr.",sr 0.31-0.56) < cummingtonite

(Xr",Cor

0.45-0.49) hornblende (Xr'",ttbt 0.30-0.58)

<

staurolite ^(Xp,e;St 0.70-0.82)

<

^garnet

(Xr",c.t

0.78-0.90, Tabie 5r2).

The garbenschiefer appear to

fall

into an aluminous and ^{a less} aluminous classifrcation;

the mineralogy of the more aluminous hornblende garbenschiefer involves

þanite,

staurolite, garnet, hornblende, chlorite, plagioclase, quartz, biotiæ, white-mica, ankerite, epidote and

rutile,

whereas less aluminous assemblages

involve

garlr.-et, cummingtonite, hornblende, chlorite, plagioclase, quartz, biotite, ankerite, epidote ^and

rutile.

The

following

sections deal

with

the compatibility relations def,rned by these assemblages.

Figure

f,1i.ftn.

^grained cummingtonite in an epitaxial relationship to foliation-forming hornblende (938-92a) widrh of view 2 mm.

?

o

c.¡

IRqJ

_4.

\

q)5

q)N

ta¡Lq)

\

sU

o 1':..

0 I

o

f /

(

\

I_o

-

f.'

() =: ^E

O-

f

significant proportions of K2O, Fe2O3, MnO, TiOZ, CO2 and minor

ZtO2,ZnO,Ct2O3

and pZOS.

In

order to graphically represent the chemistry of the phases and the compatibility relations

of

amphibolites, this list of components must be signifîcantly reduced to include only those which are

likely

to have an important bearing on the observed ^phase relations. ^Some components ^(e. g.

ZÐZ

and PZO 5, TiO2, CO2,

þO

^and Fe2O3) are generally present

in

significant proportions

in

a single phase (e.g. zircon, apatite, ilmenite or rutile, ^a carbonate phase, biotite or muscovite ^and magnetite, respectivelY). As each additional component stabilises only one phase, the variance of a given assemblage is not affected by their ^presence and they may be neglected. Other components such ^as

MnO,

ZnO andCr2O3, often occur in minor proportions in ^phases which can otherwise be described by the major elements Na2O, CaO, FeO, MgO, 41203, SiO2. Where these components occur

in

significant amounts they increase the stability of the phases they occur in wittr respect to the major element

compositional system. In ^order to

simplify

the compatibility relations, ^phases which are stabilised by a minor component ^are generally neglected (e.g. Thompson, 1954) and ^are considered to be metastable in the major-element-defined model system. Thus we ^are left with

a

list

of major components: Na2O, CaO, FeO,

MgO,

AI2O3, SiO2, HzO

(NCFMASH)'

Although this is a significant simplification of the components, the compositional system remains too complex

for

graphical representation (requiring 6 dimensions to represent the seven components). The list of components may be further simplified by assuming that other phases are

"in

excess", that is, they are always present

in

sufficient proportions that their abundance is never a

limiting

factor on any reaction. In the case of many amphibolites, quartz and plagioclase may be considered to be in excess, while an aqueous vapour is also considered to be present (or pH2O may be generally considered to be constant). This reduces the number of components to four, i.e.

CAFM.

^Although four-component, three-dimensional

compatibility diagrams may ^be represented in two dimensions, they ^are generally

difficult

to read. As calcic amphibole is also consistently present

in

amphibolites, it is considered to ^{be a} further "excess" phase

(cf.

Spear, 1978; Spear

&

Rumble, 1986), further reducing the

components to

AFM. It

must be noted that the composition of homblende and plagioclase vary over the compatibility diagram for a particular set of P-T conditions. Thus, the phase relations in kyanite-staurolite-bearing amphibolites

from

theZillertal and amphibolites from other localities

will

be represented on compatibility diagrams

with

apices AlOz2,FeO ^and MgO

(AFM) with

hornblende, plagioclase

,

qvartz and an aqueous vapour in excess. Each

assemblage is plotted separately due to ^the variation in the composition of both hornblende and plagioclase between assemblages and the results have been combined to form compatibility diagrams

for

the

Zillertal

amphibolites ^{(Fie. 5. 1} 4).

C lnpter 5 - Zillertaler AlPen - 121

FH-1M

z9M

A

Ky é,

938-127 FH-1O

GN

GN

St zlN

St

938-1 24c

chl

chl

F M

A

Ky

b

F M

Cum

Figure.

^ati

represe M

ⁱ

aqueou re

^I

study.

938-91 a

938-92a 938-51

938-52c

Chapter 5 - Zllertaler ^{AIPen - 122}